484 SECULAR PHYSICAL CHANGES. 



occurred in the chemical constitution of the air, the diminution of its an- 

 cient amount of carbonic acid, the reduction of the mean percentage of 

 its vapor of water, the increase of its oxygen, these again are changes of 

 a secular kind, the time required for the accomplishment of which is 

 wholly beyond our finite comprehension. In such a gradual advance, 

 organisms for many generations might show but little change, yet, in 

 the end, the effect must come to be profound. Indeed, all the great nat- 

 ural effects we witness are accomplished in this quiet and gradual way : 

 the traces of tempests and other catastrophes are very soon effaced, no 

 matter how violent the original commotion may have been ; but warmth, 

 and light, and moisture causes which act so gently that we might over- 

 look them are the agents which control the universal aspect of things. 

 In this, as in other respects, the strong are always the silent ; and in the 

 Secular phys- s l w l a P se f many centuries, by the gradual operation of 

 icai changes at- universal forces thus gently applied, organic forms had an 



tended by pro- . ,, j / v - 



ductions and opportunity oi accommodating or acclimating themselves to 

 extinctions. fa Q new gtate, or, if they failed to do so through some want 

 of correspondence in their structure, they gradually passed away and be- 

 came extinct. It is no argument against the transmutation of species, 

 or even of genera, that we have never witnessed such an event. We can 

 never witness the necessary combination of circumstances which should 

 bring it about, above all, as regards the needful lapse of time, the slow 

 yielding and accommodation which such a change implies. In this, as 

 in those great modifications that have occurred in the stratification of 

 the globe, the like of which has never been seen in the periods of human 

 record, our want of familiarity with them is a matter of very little mo- 

 ment. The remark of an eminent geologist applies with equal force in 

 both cases : " Changes that are rare in time become frequent in eter- 

 nity." 



But it may be said that if by external influences the successive spe- 

 Graduai change cies and genera in this manner arose, we ought to find, 

 produ^effeSs even Between those which are most closely allied, many in- 

 by abrupt crises, termediate forms ; for, since the active causes were gradual 

 in their operation, one organism should pass into another by slow de- 

 grees- so insensibly, indeed, that it would perhaps be impossible to indi- 

 cate the point at which the proper transition was made. Such an ex- 

 pectation is, however, founded upon a total misconception of the charac- 

 ter of these progresses, for a force applied for thousands of years may 

 show no effect, but at last may manifest itself by an instant crisis. Mul- 

 titudes of illustrations might be furnished of this principle ; for instance, 

 the motion of a comet may be toward the sun in a path which is almost 

 a straight line for scores of centuries, but on a sudden it assumes a curvi- 

 linear course, and accomplishes its perihelion passage in perhaps a few 



